In recent years, displays employing liquid crystals have become commonly used as the display means in personal computers, televisions and mobile phones, etc. Since these liquid crystal displays are not themselves light-emitters, display is made possible by shining a light from the rear using a backlight. For the backlight, a planar light source construction, referred to as the sidelight type or the direct type, is adopted which does not merely shine a light but meets the requirement that the entire image area be uniformly illuminated. In liquid crystal display applications used for example in notebook type personal computers where a thin, compact shape is required, there is employed the sidelight type of backlight, that is to say the type where illumination takes place from the side faces in terms of the image area. Generally speaking, in this sidelight type backlight, there is used a light guide panel system employing a light guide plate which uniformly propagates and diffuses the light to achieve uniform illumination of the entire liquid crystal display. On this light guide plate there is inscribed a pattern so that light incident from the side faces emerges in the perpendicular direction and therefore there is a non-uniform distribution of the light due to this pattern. Consequently, in this type of liquid crystal display, in order to obtain a high quality image of enhanced planar uniformity, it is necessary to place a light diffusing film on the light guide plate to make the light uniform.
The properties required of such a light diffusing film are, of course, high light diffusion, together with extremely high light transmittance. By raising the light transmittance, it is possible to utilize the light from the backlight very efficiently, so it is possible to achieve higher brightness and lower power consumption.
As examples of the light diffusing films used hitherto, there are (1) the diffusing sheet (light diffusing film) described in JP-A-4-275501 which is obtained by moulding a transparent thermoplastic resin in the form of sheet, after which processing is carried out to physically provide projections/depressions at the surface; (2) the light diffusing film described in JP-A-6-59108 which is obtained by providing a coating of a light diffusing layer, comprising a transparent resin containing fine particles, on a transparent substrate film of for example polyester resin; (3) the light diffusing plate (light diffusing film) described in JP-A-6-123802 which is obtained by the melt-blending of beads with a transparent resin and then performing extrusion; and (4) the light diffusing sheet (light diffusing film) with an islands-in-a-sea structure described in JP-A-9-311205 which is obtained by kneading at least two types of transparent thermoplastic resin. The light diffusing films (1) and (2) above, where the light diffusing effect is obtained by means of projections/indentations or a coated light diffusing layer at the film surface, are so-called surface light diffusing films. On the other hand, the light diffusing films (3) and (4) above are light diffusing films with a light diffusing component in at least the substrate interior.
Of these, the light diffusing film (2) above which is obtained by coating a light diffusing layer onto a transparent substrate film is currently the form generally used, and normally as the transparent substrate film there is employed biaxially-drawn polyethylene terephthalate film. This biaxially-drawn polyethylene terephthalate film is known to be outstanding in its mechanical strength, heat resistance, transparency and flatness, etc, and by using this film as the substrate such properties can also be manifested in the light diffusing film.
The properties demanded in the liquid crystal display components field in the future will include still higher performance and efficiency, and further reductions in thickness and weight, and in order to meet these demands there is being considered, for example, performance enhancement by surface processing and also film lamination. However, the light diffusing films obtained by (1) and (2) above have considerable surface unevenness and their lamination with other films is difficult. Furthermore, surface processing is impossible in practical terms. In the case of the light diffusing films obtained by (3) and (4) above, basically the diffusion effect is obtained by a diffusing component present in the film interior, and so the surface is flatter than in the case of the films in (1) and (2) but, nevertheless, there remains unevenness due to the presence in the vicinity of the surface layer of the beads or the thermoplastic resin from which the islands in the islands-in-a-sea structure are composed, so it is hard to say that the film flatness is adequate. Moreover, in the case of the kneading of inorganic particles or crosslinked organic particles such as beads, as in (3) above, depending on the shape and size thereof there may be blockage of the filters which are inserted in the extruder with the objective of removing impurities, and again depending of the amount of such particles added the fluidity of the resin composition at the time of melting may be impaired so that film formation is impossible.
Again, the films in (3) and (4) above are both films with considerable amounts of a diffusing component (beads for example) in the interior, so since no support is provided there is a tendency for the strength and in particular the flexural strength to be low. For example, creases are readily introduced and whitening produced, or there may be creasing or splitting at the edges at the time of cutting.
In addition, these diffusing films are for example incorporated into a backlight unit, but the component temperature becomes extremely high when the backlight is illuminated for a long period. However, the light diffusing films obtained in (3) and (4) above have poor heat resistance so, when incorporated into the backlight and used over a long period, distortion occurs, with the result that a marked luminance variation of the backlight is brought about.